One exemplary type of modular polyaxial bone screw contemplated by the present invention is a modular polyaxial pedicle screw. A pedicle is a dense stem-like bony structure that projects from the posterior of a vertebra. There are two pedicles per vertebra that connect to other structures, such as the lamina and the vertebral arch. As the result of various spinal diseases and injuries, a spinal surgeon may choose perform an interbody fusion or the like to fuse adjacent vertebrae using a bone graft and/or an implantable device, or otherwise immobilize a portion of the spine of a patient. Typically, in the interbody fusion case, the adjacent vertebrae are immobilized while the bone graft is allowed to “take,” for example, using a conventional pedicle screw system, a plate system, and/or the like.
A pedicle screw is a particular type of bone screw that is designed for implantation into a vertebral pedicle. Pedicle screws can be used in a surgical spinal fusion to add extra support and strength to the fusion while it heals. Pedicle screws are placed above and below the vertebrae that are fused. A rod is used to connect the pedicle screws, which prevents movement and allows the bone graft to heal. After the fusion is completely healed, the pedicle screws and rods can be removed.
Accordingly, surgical spinal procedures often require securing various implants to vertebrae of the spine. The pedicle screw is one such implant and is often used with other components, such as rods, that are then secured to individual pedicle screw implants in order to provide a support or fixation function between and among neighboring vertebrae. Both the rods and pedicle screws may have varying diameters and dimensions depending on patient and therapeutic needs. Due to the complex curvature and anatomy of the spine, it is sometimes difficult to align the pedicle screw and rod holder assembly with the rod, particularly when spanning multiple segments.
Traditionally, this alignment has required extensive bending and test fitting of rods to correctly approximate the rod holding portion of the pedicle screw(s). More recently, a polyaxial screw type has become widely available, which allows the rod receiving portion of the screw to pivot about the screw head. The pivoting head allows the rod holder to interface with the rod with only minimal rod contouring. These polyaxial screws are now the most common type of pedicle screw.
Current pedicle screw designs generally include: 1) a threaded bone screw shaft that is anchored into the pedicle bone of the vertebrae; 2) a rod holding member (i.e. a saddle or tulip assembly) that is attached to the head of the bone screw to receive a rod for stabilization of the spine; and 3) a set screw that interfaces with the top of the rod holder to compress the rod into the holder to form a stable construct.
These components are assembled into two primary design types: monoaxial and polyaxial screw systems. The monoaxial screws typically have a fixed angular relationship between the bone screw and rod holder (i.e. the saddle or tulip assembly). These screw designs dictate that the rod is held perpendicular to the direction of the bone screw. While these designs are strong and stable, they make it difficult to position the screw and rod properly and require a lot of rod bending to correctly approximate the rod holder and rod.
Polyaxial designs allow the rod holding member to pivot on the bone screw head, such that the rod holder can properly interface with a rod that is not perfectly perpendicular to the direction in which the bone screw is inserted. After the set screw is used to compress the rod into the rod holder, the polyaxial design locks into place and no longer allows the rod holder to pivot on the screw head.
Current designs, however, suffer from a limitation in their functionality in that the threaded screw must typically be placed in bone while the tulip assembly is coupled to the threaded screw. This leads to unnecessary complexity in the tooling and placement technique used, thereby sacrificing time and, in some cases, surgical accuracy. Thus, what is still needed in the art is a modular polyaxial bone screw that allows the threaded screw to be placed in bone separately, before the tulip assembly is engaged onto the head portion of the threaded screw.